Scotch mounts (or barn door trackers) are homemade devices to follow the stars when taking long exposures so that they will be preserved as points on the photos, and not as star trails. There are many different designs to be found on the internet, and they all probably work fine. But this is how mine turned out and it works fine for me, so I am happy with it. Some more info about star trackers can be found here.

In order to make this type of Scotch mount go with the right speed, you need to do some calculations. If the rod would be a full circle, than the mount would have to rotate along the complete circle during one sidereal day, which is the time it takes earth to rotate around its axis. It is 23 hr, 56 min, 4.1 s long, and the reason it differs from a normal 24 hour day is that a sidereal day is not compensated for the orbit that earth makes around the sun, as our normal day is. Earth rotates 365.24 times around its axis during a year when seen from the sun, but 366.24 times a year when seen from a distant star. And we want to take pictures of distant stars, so we need to calculate with sidereal days:

24 h × 365.24

duration sidereal day =

= 23 hours, 56 minutes, 4.1 seconds = 1436 minutes, 4.1 seconds

366.24

If we ignore the 4.1 seconds in the duration of a sidereal day, then the duration of a sidereal day in minutes will be 1436 minutes. In my case, I had fixed my motor and gears in such a way that the nut moving the rod would make one rotation per minute. Given the fact that the thread of the rod was 1 mm per thread, the rod needed to have 1436 threads, making it 1436 mm long. In other words, I needed to bend the rod along a circle with a circumference of 1436 mm, so that makes it possible to calculate the radius of this circle:

1436

radius =

= 228.5 mm

2 × π

So I drew a circle with a radius of 228.5 mm on a piece of paper, and bended the rod along that circle. I decided to go for a quarter of a circle with the rod, which would give me a maximum of six hours of continuous shooting, which was way more than I ever intended. Below are pictures of the final set-up.

The smaller bended rod at the front is used to compensate for the latitude at your position. You can do without this and simply put the tripod head in the right position, but that would make accurate positioning a lot harder, and the center of gravity of your system will not be directly above the tripod, giving a less stable system.

The electronics: two batteries, a switch and a regulator to adjust the speed of the motor.